In recent years, there has been a substantial amount of activity directed to increasing the life of assembled cabs and bodies for vehicles as well as other products subjected to corrosive atmospheres. Because of the intricacy of the various assembled parts and the relatively small thickness of the metal used, corrosion is becoming a substantial problem, the solution of which is now requiring a great outlay of time and money. The corrosion problem, for motor vehicles, has been compounded by an increase in the amount of salt and other chemicals being applied to the roadways in certain parts of the country. It is now a general objective of most manufacturers of motor vehicles to increase the total life of the vehicle body with respect to corrosion. At this time, it has been suggested that the steel used in the manufacture of vehicle bodies be galvanized or otherwise coated on one or both sides. Generally, the metal coating is on one side so that the other side may easily receive decorative coatings, such as paint. Also, assembled bodies are being dipped in a variety of substances and coated by electrolytic processes with materials which enhance the corrosion resistance characteristics of the body. Even with these various types of methods for attempting to reduce the corrosion of a vehicle body, additional material is required in certain inaccessible locations. This is often done by spraying additional corrosion resistant material into these areas of the body after assembly. All of these procedures have distinct disadvantages. For instance, the galvanizing of steel, or coating of steel with various metals, usually provides a coating on only one side. The painted side remains untreated. The electrical disposition process is quite expensive and must be controlled accurately for uniform results. As the process varies, it is possible to require further processing of bodies because of inferior coating by electrical disposition. These types of processes do not provide any substantial or long lasting protection at the seams between joints. In the electrical disposition process, coating is done after the parts are assembled. The same concept is true for various dipping processes. Thus, the seams of an assembled cab or body remain uncoated and can be the source of relatively rapid corrosion during adverse ambient conditions.
In an attempt to protect the seam area of an assembled body, it has been generally the practice to seal the seam at the external portions. This concept functioned on the belief that the exclusion of moisture from the seam would prevent corrosion. However, such procedures are not satisfactory since the moisture inhibiting provision of outside coatings is subsequently deteriorated to the point that moisture will migrate into the seam between assembled metal parts to cause rapid corrosion and combine with oxygen to cause corrosion.
In some instances, parts are painted before they are joined at the seams by welding. The paint forms a hardened surface which does not migrate from the welding zones in a welding process. Thus, the paint substantially increases the difficulty in obtaining a sound weld and creates inferior welding of the joint or seam. These prepainted parts are generally not successful in a welding operation for sheet metal joints or seams. To overcome this difficulty in spot welding, the paints have been provided with a powdered metal substance which reduces the resistivity of the paint in the seam area. This allowed spot welding through the seam by using the powdered metal as an electrically conductive path through the seam in the weld zone. This procedure was somewhat expensive and did not result in uniformly satisfactory spot welds. In addition, since the paint hardened on the two parts before welding of the seam, the welding operation, whether arc, gas or spot, could cause cracking of the paint. In addition, paint could crack during subsequent use along the periphery of any welded area, i.e. the spot nuggets of spot welding or the bead of gas or arc welding. Thus, this procedure of using paint for coating surfaces prior to welding of the seam is not universally used. In addition, prepainting of parts prior to welding causes handling difficulties and increases the complexity of subsequent decorative coating when the parts are to be used in components for motor vehicle cabs or bodies.
The invention of prior application Ser. No. 760,919, filed Jan. 21, 1977, relates to a new method of joining two sheet metal parts at a common seam, which method employs the selection of a corrosion resistant material having specific mechanical characteristics and then applying this material to at least one of the surfaces prior to welding of the joint. In this manner, the corrosion resistant material is sandwiched between the two sheet metal parts forming the seam to provide corrosion resistance within the seam itself. The present invention allows welding of a seam using a previously applied corrosion resistance material of the type forming a long term, self sealing corrosion resistant protection and is applicable for arc and flame or gas welding, as well as for spot welding.
In the invention of our prior application, Ser. No. 760,919, filed Jan. 21, 1977, there is disclosed the concept of seam welding two sheet metal parts together with an intermediate layer of non-hardening, high temperature corrosion resistant material. The material used in the invention has certain physical characteristics one of which is that it does not flow at relatively high temperatures up to 400.degree. F. to 440.degree. F. Since the material does not flow at high temperatures, welding can be accomplished without loss of the material from between the two surfaces forming the welded seam. The prior disclosed invention, as well as the present invention, is applicable for use in relatively thin sheet metal parts. Relatively "thin" indicates that the sheet metal is aproximately 0.010 to 0.125 in thickness. The preferred embodiment of the previously disclosed invention involves the spot welding of two relatively thin sheet metal parts formed from the normal type of steel used in production of cabs and bodies for motor vehicles. Also, such process can be used for other metal parts, such as relatively thin aluminum sheet metal parts. In the preferred embodiment of the prior process and in the present invention, the thickness is 0.020 to 0.080. However, thicker parts can be processed by this invention.